How can I merge CSS definitions in files into inline style attributes, using Perl?

I do not know of a complete, pre-packaged solution.

CSS::DOM's compute_style is subject to pretty much the same caveats as emogrifier above. That module, in conjunction with HTML::TokeParser ought to be usable to cook up something.

Update: Here is a buggy mish-mash of things:

#!/usr/bin/perl

use strict;
use warnings;

use CSS::DOM;
use File::Slurp;
use HTML::DOM;
use HTML::TokeParser;

die "convert html_file css_file" unless @ARGV == 2;
my ($html_file, $css_file) = @ARGV;

my $html_parser = HTML::TokeParser->new($html_file)
    or die "Cannot open '$html_file': $!";

my $sheet = CSS::DOM::parse( scalar read_file $css_file );

while ( my $token = $html_parser->get_token ) {
    my $type = $token->[0];
    my $text = $type eq 'T' ? $token->[1] : $token->[-1];
    if ( $type eq 'S' ) {
        unless ( skip( $token->[1] ) ) {
            $text = insert_computed_style($sheet, $token);
        }
    }
    print $text;
}

sub insert_computed_style {
    my ($sheet, $token) = @_;
    my ($tag, $attr, $attrseq) = @$token[1 .. 3];
    my $doc = HTML::DOM->new;

    my $element = $doc->createElement($tag);

    for my $attr_name ( @$attrseq ) {
        $element->setAttribute($attr_name, $attr->{$attr_name});
    }

    my $style = CSS::DOM::compute_style(
        element => $element, user_sheet => $sheet
    );

    my @attrseq = (style => grep { lc $_ ne 'style' } @$attrseq );
    $attr->{style} = $style->cssText;

    my $text .= join(" ",
        "<$tag",
        map{ qq/$_='$attr->{$_}'/ } @attrseq );
    $text .= '>';

    return $text;
}

sub skip {
    my ($tag) = @_;
    $tag = lc $tag;
    return 1 if $tag =~ /^(?:h(?:ead|tml)|link|meta|script|title)$/;
}

You can use CPAN Perl module CSS::Inliner https://metacpan.org/release/CSS-Inliner