I thought it might be interesting to benchmark the various solutions:

#!/usr/bin/env perl
use warnings;
use strict;
use Benchmark qw/cmpthese/;
use JSON::PP ();
use Cpanel::JSON::XS ();

my $IN = '"654321_1111"';
my $EXP = '654321_1111';
use constant TEST=>0;
print "Testing is ", TEST?"enabled":"disabled", "\n";

my $json_pp = JSON::PP->new->allow_nonref;
my $json_xs = Cpanel::JSON::XS->new->allow_nonref;

cmpthese(-2, {
    regex_strip => sub { # GrandFather
        (my $out = $IN) =~ s/^"|"$//g;
        $out eq $EXP or die $out if TEST;
    },
    regex_match => sub { # BillKSmith
        my ($out) = $IN =~ /^\"(.+)\"$/;
        $out eq $EXP or die $out if TEST;
    },
    tr_strip => sub { # kcott and hippo
        (my $out = $IN) =~ y/"//d;
        $out eq $EXP or die $out if TEST;
    },
    substr => sub { # syphilis and AnomalousMonk
        my $out = substr $IN, 1, -1;
        $out eq $EXP or die $out if TEST;
    },
    reverse => sub { # rsFalse
        my $out = $IN;
        for (1..2) {
            $out = reverse $out;
            chop $out;
        }
        $out eq $EXP or die $out if TEST;
    },
    json_pp => sub { # haukex
        my $out = $json_pp->decode($IN);
        $out eq $EXP or die $out if TEST;
    },
    json_xs => sub { # haukex
        my $out = $json_xs->decode($IN);
        $out eq $EXP or die $out if TEST;
    },
});

__END__

Testing is disabled
                  Rate json_pp regex_strip regex_match reverse json_xs
+ tr_strip substr
json_pp       161266/s      --        -85%        -96%    -96%    -97%
+     -97%   -99%
regex_strip  1081962/s    571%          --        -73%    -74%    -81%
+     -81%   -91%
regex_match  3989148/s   2374%        269%          --     -2%    -29%
+     -30%   -67%
reverse      4091430/s   2437%        278%          3%      --    -28%
+     -28%   -66%
json_xs      5654082/s   3406%        423%         42%     38%      --
+      -0%   -53%
tr_strip     5663601/s   3412%        423%         42%     38%      0%
+       --   -53%
substr      12107824/s   7408%       1019%        204%    196%    114%
+     114%     --
[download]

However, it's also very noteworthy that all of the solutions have their caveats, as already highlighted by AnomalousMonk and hippo:

• regex_strip will also remove unbalanced quotes, as in q{"foo} becomes q{foo}
• regex_match will also remove quotes from things that - depending on your input format - may not be syntactically valid, like q{"foo"bar"} becomes q{foo"bar}
• tr_strip will remove all quotes in the string no matter where they appear, i.e. q{"foo""bar"} becomes q{foobar}
• substr and reverse will remove the first and last character unconditionally, i.e. q{foobar} becomes q{ooba}
• json_pp and json_xs only apply if the input format is actually JSON or something that can be parsed as if it were, e.g. they will refuse to parse q{"foo"bar"}

This underscores what others have already said: it would be better if you could better specify your input format.

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