I would like to add a few things I've learned from experience.
I would change:

my $extlist = join('|', @extensions);
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to

my $extlist = join('|', map {quotemeta} @extensions);
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in case members of @extensions contains some metacharacters. There might be an extension file.(myextension)
I'd also change:

my $re = qr/some_text=".*\.(?:$extlist)"/;
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to

my $re = qr/some_text=".*\.(?:$extlist)"$/;
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because you never know if someone has a file named something like "file.123".backup
It doesn't matter in these simple examples but when you try using it in the real world you never know what you will get. I try to think of pathological cases because most users are, well, pathological. Someone else will probably have some better suggestions on my code. They will probably be speaking from experience (of pathological users).

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flounder